Bugs in my miso!

I opened a pack of white miso I haven't used for a while last night to find the top layer had gone all dark brown and crumbly, like compost, and there were pale little seeds in it. At first I thought the miso had spored or something (?!?) but on closer inspection the "seeds" turned out to be little bugs all wriggling about!

What are they? Are they harmful? Are they like the little guys you sometimes get crawling though rice that's been sitting too long on the shelf?